<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        /*当前选中的元素，包含class="content"*/
        div.content{
            color: red;;
        }
        /*后代选择器*/
        div .nav{
            color: blue;
        }
    </style>
</head>
<body>
<div>
    aaaaa
    <ul class="content nav">bbbb</ul>
</div>
<div class="content">
    ccccc
    <ul>ddddd</ul>
</div>
</body>
</html>